The present invention relates to process control instruments. More specifically, the present invention relates to the process coupling used to couple a pressure transmitter to process fluid.
Industrial processes such as oil refineries, chemical manufacturing plants, etc., use remote sensors known as xe2x80x9ctransmittersxe2x80x9d to sense various process variables such as pressure, temperature, flow, etc. The sensed process variables are used to monitor and/or control operation of the process.
Transmitters which sense pressure of a process fluid use a pressure sensor which typically is coupled to at least one isolation diaphragm. The transmitters attach to a flange and include an opening which is aligned to receive process fluid from a passageway in the flange. The isolation diaphragm is positioned in the opening of the transmitter and isolates the pressure sensor from the process fluids being sensed. This prevents the sensor from being damaged or corroded by the process fluid. Pressure is transferred from the isolation diaphragm to the pressure sensor through a substantially incompressible isolation xe2x80x9cfill fluidxe2x80x9d which is carried in a capillary passageway. U.S. Pat. No. 4,833,922 entitled MODULAR PRESSURE TRANSMITTER and U.S. Pat. No. 5,094,109 entitled PRESSURE TRANSMITTER WITH STRESS ISOLATION DEPRESSION are examples of pressure transmitters of this type.
Pressure transmitters frequently include a weld to secure the isolation diaphragm to the transmitter body. A weld ring can be used which is welded to the transmitter body and to the isolation diaphragm, thus securing the isolation diaphragm to the transmitter body. The weld ring can also be used to support a seal, such as an O-ring. When the transmitter is attached to the process flange by an attachment force, for example if the transmitter is bolted to the flange, the seal and weld ring are compressed against the flange to prevent process fluid from leaking past the opening.
The attachment force which compresses the weld ring against the flange tends to deform or deflect portions of the weld ring and transmitter body. Further, the attachment force causes stress in the isolation diaphragm which leads to deformation or deflection of the isolation diaphragm. When attempting to make highly accurate pressure measurements, this deformation or deflection of the isolation diaphragm can cause inaccuracies in the measured pressure.
A process transmitter for measuring a process pressure includes a pressure sensor in a sensor housing having a face configured to be exposed to process fluid. A fill fluid capillary tube couples fluid from the pressure sensor to the face of the sensor body. An isolation diaphragm isolates the fill fluid from the process fluid. A weld seals the isolation diaphragm to the face and a process fluid seal seals a process fluid connection to the face of the sensor housing to thereby couple process fluid to the isolation diaphragm. The process fluid seal is spaced apart from the weld to reduce stress on the isolation diaphragm from the process fluid seal.